Support 'mixed' IPv6 representations

[Porges]

RFC4291 specifies a "mixed" IPv6 syntax that is not currently supported:

3. An alternative form that is sometimes more convenient when dealing
   with a mixed environment of IPv4 and IPv6 nodes is
   x:x:x:x:x:x:d.d.d.d, where the 'x's are the hexadecimal values of
   the six high-order 16-bit pieces of the address, and the 'd's are
   the decimal values of the four low-order 8-bit pieces of the
   address (standard IPv4 representation). Examples:

    `0:0:0:0:0:0:13.1.68.3`
   
    `0:0:0:0:0:FFFF:129.144.52.38`
   

   or in compressed form:

    `::13.1.68.3`
   
    `::FFFF:129.144.52.38`
   

[andrewthad]

I would gladly take a PR adding support for this.

[Porges]

There you go 😁